There is frequently a need to very accurately displace a fluid. For example, in apparatus used in the pulsating bubble technique for evaluating the surface tension of a pulmonary surfactant requires that precise fluid quantities be displaced in order to properly evaluate the surface tension of the fluid being displaced. Such an apparatus is shown in Prog. Resp. Res., vol. 15, pp. 57-61 and also J. Appl. Physiol.: Respirat. Eviron. Exercise Physiol. 43 (2): pp 198-203, 1977. It has been found that in the prior apparatus that linear movement of a fluid displacing device could not be translated directly into the quantity of fluid being displaced, primarily due to seal movement. In addition, it has been found that fluid to one side of the seal could contaminate fluid to the other side of the seal.